In a wireless communications system, the communications system's capacity may be significantly improved when a transmitter (also referred to as a base station) has full or partial knowledge of a channel over which it will be transmitting. Information related to the channel may be referred to as channel state information (CSI). CSI may be obtained by the transmitter over a reverse feedback link through the channel. A receiver (also referred to as a mobile user end or mobile station or user equipment) of the transmissions made by the transmitter may transmit CSI back to the transmitter over the reverse feedback link through the channel.
In MIMO (multiple input multiple output) wireless communications, channel state information can be estimated at the wireless receiver and fed back to the wireless transmitter, so that the transmitter can adapt its transmit scheme to the wireless channel. Such feedback is usually achieved through a codebook, which serves as a common dictionary of the current channel condition to the wireless transmitter and receiver.
However, in practice, changes in the wireless channel can be much more severe than what can be accommodated by a static codebook. Using a dynamic codebook overcomes this problem but introduces the problem of increased complexity.